1. Field of the Invention
The present invention relates to a novel adhesive composition and process for applying a styrenic based adhesive and cement coatings to thermoplastic polymer surfaces, particularly to toy articles formed of resin materials. Specifically, the present invention relates to a cyanoacrylate adhesive composition having superior bonding and hot-cold cycle resistance characteristics, particularly suited for unfriendly toy play environments.
2. Description of the Related Art
Children's toys and dolls, in particular, have a history dating back into antiquity. Generally, the technology surrounding the manufacture of dolls centers on creation of an attractive doll face, miniturized application of conventional clothes, manufacture of a doll body using plush or other sculptural techniques, and the simulation of the hair with sculptural elements, fibers, or other techniques. In the case of doll faces, the doll maker has a wide panoply of possible constructions and manufacturing techniques at his disposal. Traditional techniques involve the use of plaster-like or hard plastics material to cast a doll face. In time, however, soft rubber materials such as styrene-butadiene rubber (SBR) and styrene-butadiene-styrene block copolymer (Kraton) elastomers have come to replace plaster and hard plastics in the production of doll heads because of their realistic look and feel.
The soft rubber doll faces include all the normal features of a human figure simulated by the doll including the entire head, including well formed lips, teeth, cheeks, nose, chin, ears, and forehead. After the injection molding of an elastomeric doll face, it is finished by applying a dye or other similar material to give the face a natural skin color. Thereafter the soft rubbery doll's head must be joined to a body portion comprised of a torso and limbs to complete a full bodied toy doll item. This is generally accomplished by gluing, sealing or cementing the soft eleastomeric head potion to the headless body section generally composed of a hard or firm plastic. In selecting an adhesive for the purpose of affixing the dolls' head two considerations are important: (1) that the adhesive form a firm and effective bond and seal between the soft elastomer and the harder resin substrate surface and (2) that the adhesive composition be relatively quick drying so as to accommodate expeditious doll manufacturing processes. The principle outlined here can be applied to any toy parts requiring laminating or bonding affixation of parts. The present invention provides a cyanoacrylate adhesive which provides superior bonding and rapid curing or drying.
It is well known that alpha.-cyanoacrylate (also known as 2-cyanoacrylate) adhesives polymerize and cure rapidly in the presence of trace water absorbed on the surface of a material to be bonded, affording an extremely strong bond in a short time for almost all materials except some inert materials such as polyethylene and Teflon. Because of such a superior property, alpha-cyanoacrylate adhesives are widely used as instantaneous adhesives for a room temperature setting as one-part type adhesives for the bonding of metals, plastics, rubber, wood, etc. However, since the cyanoacrylate polymer is a brittle polymer, cyanoacrylate adhesives have the drawback that they are inferior in peeling bond strength and impact resistance. This problem has been generally remedied by modifying the cyanoacrylate adhesive compositions with various elastomers, core-shell polymers, adhesives and reinforcing agents. The present invention carries this modification concept one step further in employing styrene block co-polymers in adhesive applications for toy manufacturing.
As indicated above, the class of the styrenic elastomeric materials commonly used as the molding material for toy doll faces and the harder structural portions of toy articles are A-B-A type block polymers such as styrene-butadiene-butylene copolymer-styrene or styrene-butadiene-styrene, manufactured by Shell and sold under the trademark Kraton. In the case of a toy having a hard surface comprised of a rigid or semirigid A-B-A styrene based block copolymer, a compatible elastomeric A-B-A styrene copolymer in the form of styrene-ethylene-butylene-styrene, (SEBS, sold under the trademark Kraton-G) has been found to be an exceptional modifier of the alpha-cyanoacrylate adhesives to render them useful as glues, cements, or sealants to bond mechanical parts in in toy items.
For adhesives used to bond toy mechanical parts and like structures which are used in rugged or otherwise unfriendly environments it is considered necessary to have not only a high bond strength in the normal state of use but also a high adhesive force to endure the heavy handling of child's play. Moreover, the bonded mechanical parts must even be capable of withstanding extreme or sudden changes in temperature caused by leaving the toy item outdoors or immersing the toy item in hot and cold water, e.g. a child subjecting a doll to a sink bath. Conventional cyanoacrylate adhesive compositions are not generally satisfactory in meeting these demands as adhesives in toy items.
It has been found that a cyanoacrylate adhesive composition containing certain styrenic block copolymers and certain solvents results in an adhesive which not only possesses high shear adhesive force, peeling bond strength and impact adhesive force but also exhibits an outstanding hot-cold cycle resistance and heat resistance.